Alkene decolourisation of bromine water. Image 1 of 3. Start of the reaction of bromine water (Br2, brown) with ethene (C2H4, a gaseous alkene) in a test tube. The ethene will be bubbled into the bromine water through the glass tube seen in the test tube. This is an electrophilic addition reaction that will decolourise the bromine. The bromine adds across the ethene double bond, producing the colourless 1,2-dibromoethane (C2H4Br2). This reaction is used to demonstrate the reactivity of the alkene double bond. In contrast, single-bonded alkanes do not react with bromine unless ultraviolet light is used to initiate the reaction. For a sequence of the decolourisation, see images A500/725-727.